The present invention relates to the field of small diameter holes drilled in hard geologic formations such as rock. More particularly, the invention relates to a portable apparatus for efficiently creating holes such as seismic shotholes in rock.
Rock holes are drilled for excavation blasting, mining operations, and many other purposes. For example, explorative searches for hydrocarbons, minerals, and other products require the physical penetration of geologic formations. Seismic operations typically detonate explosive charges to generate shock wave source signals for penetrating subsurface geologic formations. The shock waves are reflected from subsurface geologic structures and interfaces and the reflected energy is detected with sensors such as geophones at the surface. Transducers reduce the reflected energy into signals which are recorded for processing.
In many land-based geophysical seismic operations, vibrator trucks contact the soil and discharge energy into subsurface geologic formations. However, survey regions frequently comprise mountainous, tropical, or other regions inaccessible to seismic trucks. Because of accessibility constraints and the large source energy provided by explosive materials, explosive charges detonated in shot-holes provide a preferred source of seismic source energy. Shot holes up to four wide and between ten and thirty meters deep are drilled in surface geologic formations. Explosive charges are placed in the bottom of the shot-hole and are detonated to generate shock waves transmitted into the subsurface geologic formations.
Seismic shot-holes require different parameters than excavation blast holes because the objective of shot-holes is not to displace or fracture rock, but to efficiently transfer elastic shock wave energy downwardly into subsurface geologic formations. Accordingly, shot-hole equipment and drilling techniques are relatively specialized. As representative examples, U.S. Pat. No. 3,939,771 to McReynolds (1976) disclosed a seismic explosive charge loader and anchor. U.S. Pat. No. 4,278,025 to McReynolds (1981) disclosed a seismic explosive charge loader having a spring anchor for retaining the charge in the borehole. U.S. Pat. No. 4,546,703 to Thompson (1985) disclosed a device for placing an explosive charge into a borehole. U.S. Pat. No. 4,660,634 to Johnson, Jr. (1987) disclosed an automatic drill pipe breakout especially suited for geophysical seismic drilling. U.S. Pat. No. 5,281,775 to Gremillion (1994) disclosed a vibration hole forming device for shot-hole drilling from a lightweight drill.
The diameter of conventional explosive charges is smaller than the shot-hole diameter to facilitate placement of the explosives into the lower shot-hole end. The resulting annulus between the explosive charge and the shot-hole wall does not efficiently couple the shock wave energy to the subsurface geologic formations. Moreover, a large portion of the shock wave energy is discharged upwardly through the shot-hole because of the relatively low resistance provided by the open hole. To limit this energy loss, plugs are placed in the shot-hole as shown in U.S. Pat. No. 4,066,125 to Bassani (1978). U.S. Pat. No. 4,736,796 to Arnall et al. (1988) disclosed other techniques for sealing shot-holes with cement, gravel, and bentonite.
Hard rock drills use compressed air to drive a hammer in mining and tunneling operations. A rotary percussion hammer drives a narrow, hexagonal shaped bit into the rock to pulverize the rock and to create the rock hole. Such drills are not useful at distances from the rock surface because such drills jam within the rock hole and become stuck. This sticking is caused by variations in the hole annular area due to the hexogonal bit shape, by the tendency for rock particles to lodge against the exterior bit edges, and by insufficient airflow velocity through the hexagonal bit. The failure to remove rock particles from the hole generated increases the probability of bit sticking within the hole and the loss in efficiency caused by such factors.
Regional seismic operations require multiple shothole locations for a seismic survey, and large surveys can require thousands of shotholes. The average cost for each shothole multiplied by the number of shotholes significantly determines the economic efficiency of the survey and the data sets obtainable from a survey design. A need exists for improved techniques for efficiently creating holes such as seismic shotholes in areas inaccessible by heavy equipment.
The present invention provides an apparatus engagable with a portable rock drill and compressed air supply for forming a hole in rock. The apparatus comprises a rock bit having a selected radial dimension for breaking the rock into rock cuttings to form the rock hole, a connector attached to the bit and to the rock drill wherein the connector includes a substantially tubular shaft having an exterior radial dimension less than the selected radial dimension of the rock bit, and an aperture through the connector for receiving compressed air from the rock drill and for conveying the compressed air to the rock bit for transporting rock cuttings from the rock hole.
In different embodiments of the invention, the connector can include a tool adapter having a port for receiving a compressed air supply, a drill pipe body attached to the tool adaptor, or a tool crossover for attaching a drill pipe body to a tool adaptor, or a drill pipe end for attachment to the rock bit.